Computed tomography is an imaging method employing tomography. In computed tomography, digital processing is used to generate a cross-sectional image of the inside of an object from a series of two-dimensional x-ray projection images taken around a single axis of rotation. A three-dimensional image can be created by stacking adjacent cross-sectional images or by directly generating an image from a series of two-dimensional x-ray projection images.
Helical (or spiral) cone beam computed tomography is a type of three dimensional computed tomography in which the source (usually of x-rays) describes a helical trajectory relative to the object while a two dimensional array of detectors measures the transmitted radiation on part of a cone of rays emanating from the source. In helical cone beam x-ray computed tomography scanners, the source and array of detectors move on a rotating gantry while the patient is moved axially at a uniform rate. Earlier x-ray computed tomography scanners imaged one slice at a time by rotating the source and a one-dimensional array of detectors while the patient remained static. The helical scan method reduces the measurement time for a given resolution. This is achieved, however, at the cost of greater mathematical complexity in the reconstruction of the image from the measurements.
Computed tomography produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the x-ray beam. Although historically the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
In some applications, it is desired to generate images of small structures within a larger object. For example, the visualization of changes in small trabecular structures, required to understand bone physiology, requires the acquisition of complete images at high resolution. Trabecular structures range in size from 25 to 200 μm. However, such visualization presently requires exposure of the patient to increased doses of radiation. Therefore, because of such increased radiation requirements, high-resolution images of trabecular structures using computed tomography has been limited mostly to in-vitro analysis of bone biopsies.
Accordingly, the need still exists in the art for a focused high-resolution computed tomography scanner and method of scanning that reduces the radiation dose while maintaining the high resolution needed to image small structures.